In order to address the aging of the population composition and the demand for social security and safety, there is a quickly increasing demand for camera modules aiming at applications such as monitor cameras, surveillance cameras and home monitoring cameras. For these camera modules, there is an increasing demand for lowering the price for reducing the system cost and for reducing the size in order to enhance the degree of freedom for mounting cameras.
Patent Document No. 1 discloses a camera module that addresses the demand for reducing the size. Specifically, as a measure for reducing the size, it has been proposed to form a wafer-level camera module including a lens and an imaging element chip integral with each other, thereby reducing the size of the camera module.
In order to address the demand for low cost, wafer-level camera modules of Patent Document No. 2 are obtained by severing, into individual camera modules, a structure including a lens wafer and an imaging element wafer bonded together via a bonding section.
Camera modules using no lens are known as pinhole cameras. According to Patent Document No. 3, an imaging element is provided on the bottom portion of a hollow package casing, and a protection plate is formed on top of the casing, wherein a pinhole is formed from a transparent material in the central portion of the protection plate, and the protection plate other than the pinhole is formed by a light-blocking material. The disclosure states that the viewing angle is defined by changing the distance from the pinhole to the sensor.
Patent Document No. 4 shows a method in which a projection of a cover casing having a pinhole formed therein is pressed against the end portion of an imaging element chip so that the pinhole is positioned at the center of the photosensitive area of the imaging element.
Typically, when the opening edge of the lens system is sharp, the resultant image will be unnatural for high frequency components, referred to as false resolution, due to the influence of optical interference. In order to prevent this, there is an approach known in the art for improving false resolution, which uses a filter, called an apodization filter, configured so that the amount of light to be transmitted therethrough decreases gradually away from the optical axis in the direction perpendicular to the optical axis.
The apodization filter is preferably a flat plate filter that is not powerful, taking into consideration the ease of handling, the degree of freedom in optical design, the influence on the performance of the overall optical system, etc. This is why the configuration of Patent Document No. 5 is obtained by attaching together a plano-concave lens made of an ND (neutral density) glass and a plano-convex lens made of a glass that has the same refractive index as the ND glass. The provision of an apodization filter as described above requires improvements in the filter processing precision, the assembly precision, and the refractive index precision, thus substantially increasing the difficulty.